Paleomagnetic and oxygen isotopic records from the upper Pliocene marine succession in the Western North Pacific: insights into the Mammoth and Kaena subchrons

Robust chronological frameworks that integrate paleomagnetic and oxygen isotope records are essential to stratigraphic correlation across diverse geological records. However, such records remain scarce for the Late Pliocene. This issue is particularly evident in the western North Pacific region around Japan, where many sedimentary basins formed below the carbonate compensation depth, yielding fewer well-preserved carbonates than those from the Atlantic Ocean. In this study, we present a high-resolution integrated chronostratigraphy of paleomagnetic and stable oxygen isotope (δ 1 8 O) records for the Late Pliocene (3.4–2.9 Ma), based on data from the marine sedimentary succession of the Chikura Group in the southernmost part of the Boso Peninsula, central Japan. In addition, we present a detailed paleomagnetic record of the upper boundary of the Kaena subchronozone and examine the behavior of the geomagnetic field during the polarity transition. Based on the benthic foraminiferal δ 1 8 O record, we developed an isotope stratigraphy corresponding to Marine Oxygen Isotope Stages (MIS) MG5 to G14, indicating that the lower Mammoth and upper Kaena boundaries correspond to MIS MG1 and G21, respectively. The relative paleointensity (RPI) record shows distinct fluctuations that correlate well with RPI variations observed in globally distributed deep-sea cores. A pronounced RPI minimum at 2.95–2.92 Ma, accompanied by unstable magnetic directions, is interpreted as a possible geomagnetic excursion or cryptochron. The upper Kaena polarity transition is characterized by rapid directional changes during an interval of reduced geomagnetic field intensity. The paleomagnetic and oxygen isotope records presented in this study provide new constraints on geomagnetic field behavior during polarity transitions and improve understanding of their timescale and structure.